1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to driving control of a fan for cooling a sheet.
2. Description of the Related Art
Conventionally, in image forming apparatuses such as a copying machine and a printer for performing image formation using an electrophotographic method, a toner image is transferred onto a sheet, and the sheet is then conveyed to a fixing device to fix the toner image onto the sheet, to form an image on the sheet. Further, some image forming apparatuses include a two-sided image formation mode in which a reversing unit reverses a sheet having an image formed on its front surface, and a re-conveyance portion then conveys the sheet to an image forming portion again to form an image on its back surface, to perform image formation on both the surfaces of the sheet.
In the conventional image forming apparatus, a sheet is discharged onto a sheet discharge tray after the fixing. At that time, the sheet may not be sufficiently cooled. In this case, a phenomenon in which sheets discharged onto the sheet discharge tray are affixed together by fused toner (hereinafter referred to as a blocking phenomenon) may occur.
As this measure, a cooling unit for contacting a sheet with cooling air in a sheet stacking direction, to decrease the temperature of the sheet discharged onto a sheet discharge tray (see U.S. Patent No. 2007/0196152). Alternatively, a fan is operated and stopped, as needed, by disposing the fan above a sheet discharge tray while determining whether stacked sheets are to be air-cooled (see Japanese Patent Application Laid-Open No. 2007-079310).
When thus configured, the image forming apparatus can stop the fan under conditions that no blocking phenomenon occurs on the sheet discharge tray, for example, when the number of stacked sheets is small or a distance between sheets is wide, so that noise and power consumption can be minimized.
FIG. 6 illustrates, in a conventional image forming apparatus in which a fan 402 cools sheets stacked on a sheet discharge tray 310, the flow of air by the fan 402. In the image forming apparatus, a sheet S1 having an image formed on its one surface (first surface) by passing through a fixing device 300 is generally discharged onto the sheet discharge tray 310.
When the sheet S1 is thus discharged onto the sheet discharge tray 310, the fan 402 causes cooling air indicated by an arrow to flow between the discharged sheet S1 and an already discharged sheet Sa, as illustrated in FIG. 6A. The occurrence of a blocking phenomenon in which sheets are affixed together can be prevented by flowing cooling air between the discharged sheet S1 and the already discharged sheet Sa.
In a two-sided image formation mode for forming images on both front and back surfaces of a sheet, an image is formed on a first surface of a sheet S1, and the sheet S1 that has passed through the fixing device 300 is guided into a guide path 301. The sheet S1 guided into the guide path 301 is guided into a reversing path 305 by forward rotation of reversing rollers 302 and 303, and is then fed out in an opposite direction to a direction in which the sheet S1 is fed in with its trailing edge at the head by backward rotation of the reversing roller 303.
The sheet S1 fed out with its leading and trailing edges in a sheet conveyance direction thus reversed is fed to a two-sided conveyance path 306 while being guided by a guiding member (not illustrated), and is then conveyed to an image forming portion again so that a toner image is transferred onto a back surface of the sheet S1. The sheet S1 having the image formed on its back surface by passing through the fixing device 300 is discharged onto the sheet discharge tray 310. The occurrence of a blocking phenomenon can be prevented by flowing cooling air between the sheet S1 thus discharged onto the sheet discharge tray 310 and the already discharged sheet Sa.
The image forming apparatus includes an inversed discharge mode for discharging a sheet that has passed through the fixing device 300 onto the sheet discharge tray 310 with its front and back surfaces and its leading and trailing edges in the sheet conveyance direction reversed. In the inversed discharge mode, the sheet S1 is guided into the guide path 301, then conveyed to an inversed discharge path 307 by backward rotation of the reversing rollers 302 and 303 and switching of the guiding member, and discharged onto the sheet discharge tray 310.
In the conventional image forming apparatus, however, the guide path 301 is branched downward, as illustrated in FIG. 6. Therefore, in the inversed discharge mode, the sheet S1 that has passed through the fixing device 300 passes through the curved guide path 301. In this case, the sheet S1 is curled due to downward curvature of the guide path 301.
When reversed and discharged, the sheet S1 thus curled enters an upward curled state in which its leading and trailing edges in a sheet discharging direction are curved upward in the sheet discharge tray 310 because its front and back surfaces are reversed. When the sheet S1 is discharged onto the sheet discharge tray 310 after passing through the inversed discharge path 307, the sheet S1 is cooled while being affected by the curvature of the guide path 301 immediately after passing through the fixing device 300. Therefore, the sheet S1 is not easily affected by curvature of the inversed discharge path 307.
When the fan 402 causes cooling air to flow between the discharged sheet S1 and the already discharged sheet Sa in this state, cooling air indicated by an arrow flows under a lower surface of the sheet S1 in the upward curled state discharged onto the sheet discharge tray 310, as illustrated in FIG. 6B. Thus, the sheet S1 floats and collides with the subsequent sheet S2. As a result, sheet stacking properties on the sheet discharge tray 310 are deteriorated.
If the guide path 301 is branched upward, the sheet 51 is curled due to upward curvature of the guide path 301. When reversed and discharged, the sheet S1 thus curled enters a downward curled state in which its leading and trailing edges in the sheet discharging direction are curled downward in the sheet discharge tray 310 because its front and back surfaces are reversed.
When the fan 402 causes cooling air to flow between the discharged sheet S1 and the already discharged sheet Sa in this state, the discharged sheet S1 in the downward curled state receives the cooling air at the leading edge in the sheet discharging direction, and thus is conveyed downstream in the sheet discharging direction of a normal sheet discharge position. Therefore, the leading edge in the sheet discharging direction of the sheet S1 hangs from the sheet discharge tray 310 so that the sheet S1 may not be able to return to its proper stacking position by inclination of a stacking face of the sheet discharge tray 310.
If a decurl unit for correcting upper curling and lower curling of the sheet S1 is provided to secure sheet stacking properties, the image forming apparatus increases in size and cost.